Introduction to the "React" layer#

Some admin modules render their UI with React, a popular JavaScript library created by Facebook. For these sections, rendering happens via client side scripts that create and inject HTML declaratively using data structures.

Even within sections that are not primarily rendered in react, several React components may be injected into the DOM.

There are some several members of this ecosystem that all work together to provide a dyanamic UI. They include:

• ReactJS - A JavaScript UI library
• Redux - A state manager for JavaScript

All of these pillars of the frontend application can be customised, giving you more control over how the admin interface looks, feels, and behaves.

First, a brief summary of what each of these are:

React#

React's job is to render UI. Its UI elements are known as "components" and represent the fundamental building block of a React-rendered interface. A React component expressed like this:

import React from 'react';
// ...

  <PhotoItem size={200} caption="Angkor Wat" onSelect={openLightbox}>
    <img alt="" src="path/to/image.jpg" />
  </PhotoItem>;

Might actually render HTML that looks like this:

<div class="photo-item">
    <div class="photo" style="width:200px;height:200px;">
        <img src="path/to/image.jpg">
    </div>
    <div class="photo-caption">
        <h3><a>Angkor Wat/a></h3>
    </div>
</div>

This syntax is known as JSX. It is transpiled at build time into native JavaScript calls to the React API. While optional, it is recommended to express components this way.

Recommended: react dev tools#

The React Dev Tools extension available for Chrome and Firefox is critical to debugging a React UI. It will let you browse the React UI much like the DOM, showing the tree of rendered components and their current props and state in real time.

Redux#

Redux is a state management tool with a tiny API that affords the developer highly predictable behaviour. All of the application state is stored in a single object, and the only way to mutate that object is by calling an action, which is just a simple object that describes what happened. A function known as a reducer mutates the state based on that action and returns a new reference with the updated state.

The following example is taken from the Redux GitHub page:

// reducer
function counter(state = 0, action) {
  switch (action.type) {
    case 'INCREMENT':
      return state + 1;
    case 'DECREMENT':
      return state - 1;
    default:
      return state;
  }
}

const store = createStore(counter);
// subscribe to an action
store.subscribe(() => {
  const state = store.g.etState();
  // ... do something with the state here
});

// Call an action - in this case increment the state from 0 to 1
store.dispatch({ type: 'INCREMENT' });

Recommended: redux devtools#

It's important to be able to view the state of the React application when you're debugging and building the interface.

To be able to view the state, you'll need to be in a dev environment and have the Redux Devtools installed on Google Chrome or Firefox, which can be found by searching with your favourite search engine.

For more information#

This documentation will stop short of explaining React and Redux in-depth, as there is much better documentation available all over the web. We recommend:

• The Official React Tutorial
• Build With React
• Getting Started with Redux

Build tools and using Silverstripe CMS react components#

Silverstripe CMS includes react, redux, and many other thirdparty dependencies already, which are exposed using webpack's expose-loader plugin for you to use as webpack externals.

There are also a lot of React components and other custom functionality (such as the injector, mentioned below) available for reuse. These are exposed in the same way.

The recommended way to access these dependencies is by using the @silverstripe/webpack-config npm package. The documentation in the readme for that package explains how to use it.

If you are not using webpack to transpile your JavaScript, see if your build tooling has an equivalent to webpack's externals configuration. Alternatively, instead of importing these dependencies, you can access them on the window object (for example the injector module is exposed as window.Injector).

The Injector API#

Much like Silverstripe CMS's Injector API in PHP, the client-side framework has its own implementation of dependency injection known as Injector. Using Injector, you can register new services, and transform existing services.

Injector is broken up into three sub-APIs:

• Injector.component for React UI components
• Injector.reducer for Redux state management
• Injector.form for forms rendered via FormSchema.

The frontend Injector works a bit differently than its backend counterpart. Instead of overriding a service with your own implementation, you enhance an existing service with your own concerns. This pattern is known as middleware.

Middleware works a lot like a decorator. It doesn't alter the original API of the service, but it can augment it with new features and concerns. This has the inherent advantage of allowing all thidparty code to have an influence over the behaviour, state, and UI of a component.

A simple middleware example#

Let's say you have an application that features error logging. By default, the error logging service simply outputs to console.error. But you want to customise it to send errors to a thirdparty service. For this, you could use middleware to augment the default functionality of the logger.

// LoggingService.js

/* eslint-disable-next-line no-console */
const LoggingService = (error) => console.error(error);

export default LoggingService;

Now, let's add some middleware to that service. The signature of middleware is:

const middleware = (next) => (args) => next(args);

Where next() is the next customisation in the "chain" of middleware. Before invoking the next implementation, you can add whatever customisations you need. Here's how we would use middleware to enhance LoggingService.

import thirdPartyLogger from 'third-party-logger';

const addLoggingMiddleware = (next) => (error) => {
  if (error.type === LoggingService.CRITICAL) {
    thirdpartyLogger.send(error.message);
  }
  return next(error);
};

Then, we would create a new logging service that merges both implementations.

import LoggingService from './LoggingService';
import addLoggingMiddleware from './addLoggingMiddleware';

const MyNewLogger = addLoggingMiddleware(LoggingService);

We haven't overridden any functionality. LoggingService(error) will still invoke console.error, once all the middleware has run. But what if we did want to kill the original functionality?

import thirdPartyLogger from 'third-party-logger';
import LoggingService from './LoggingService';

const addLoggingMiddleware = (next) => (error) => {
  // Critical errors go to a thirdparty service
  if (error.type === LoggingService.CRITICAL) {
    thirdPartyLogger.send(error.message);
  }
  // Other errors get logged, but not to our thirdparty
  else if (error.type === LoggingService.ERROR) {
    next(error);
  }
  // Minor errors are ignored
  else {
    // Do nothing!
  }
};

Registering new services to the Injector#

If you've created a module using React, it's a good idea to afford other developers an API to enhance those components, forms, and state. To do that, simply register them with Injector.

// my-public-module/js/main.js
import Injector from 'lib/Injector';

Injector.component.register('MyComponent', MyComponent);
Injector.reducer.register('myCustom', MyReducer);

Services can then be fetched using their respective .get() methods.

const MyComponent = Injector.component.get('MyComponent');

Transforming services using middleware#

Now that the services are registered, other developers can customise your services with Injector.transform().

// someone-elses-module/js/main.js
Injector.transform(
  'my-transformation',
  (updater) => {
    updater.component('MyComponent', MyCustomComponent);
    updater.reducer('myCustom', MyCustomReducer);
  }
);

Much like the configuration layer, we need to specify a name for this transformation. This will help other modules negotiate their priority over the injector in relation to yours.

The second parameter of the transform argument is a callback which receives an updaterobject. It contains four functions: component(), reducer(), form.alterSchema() and form.addValidation(). We'll cover all of these in detail functions in detail further into the document, but briefly, these update functions allow you to mutate the DI container with a wrapper for the service. Remember, this function does not replace the service - it enhances it with new functionality.

Helpful tip: name your component middleware#

Since multiple enhancements can be applied to the same component, it will be really useful for debugging purposes to reveal the names of each enhancement on the displayName of the component. This will really help you when viewing the rendered component tree in React Dev Tools.

For this, you can use the third parameter of the updater.component function. It takes an arbitrary name for the enhancement you're applying.

// module-a/js/main.js
(updater) => updater.component('TextField', CharacterCounter, 'CharacterCounter');

// module-b/js/main.js
(updater) => updater.component('TextField', TextLengthChecker, 'TextLengthChecker');

Controlling the order of transformations#

Sometimes, it's critical to ensure that your customisation happens after another one has been executed. To afford you control over the ordering of transforms, Injector allows before and after attributes as metadata for the transformation.

// my-module/js/main.js
Injector.transform(
  'my-transformation',
  (updater) => {
    updater.component('MyComponent', MyCustomComponent);
    updater.reducer('myCustom', MyCustomReducer);
  },
  { after: 'another-module' }
);

before and after also accept arrays of constraints.

Injector.transform(
  'my-transformation',
  (updater) => updater.component('MyComponent', MyCustomComponent),
  { before: ['my-transformation', 'some-other-transformation'] }
);

Using the * flag#

If you really want to be sure your customisation gets loaded first or last, you can use * as your before or after reference.

Injector.transform(
  'my-transformation',
  (updater) => updater.component('MyComponent', FinalTransform),
  { after: '*' }
);

Injector context#

Because so much of UI design depends on context, dependency injection in the frontend is not necessarily universal. Instead, services are fetched with context.

example:

const CalendarComponent = Injector.get('Calendar', 'AssetAdmin.FileEditForm.StartDate');

Likewise, services can be applied for specific contexts.

Injector.transform('my-transform', (updater) => {
  // Applies to all text fields in AssetAdmin
  updater.component('TextField.AssetAdmin', MyTextField);

  // Applies to all text fields in AssetAdmin editform
  updater.component('TextField.AssetAdmin.FileEditForm', MyTextField);

  // Applies to any textfield named "Title" in AssetAdmin
  updater.component('TextField.AssetAdmin.*.Title', MyTextField);

  // Applies to any textfield named "Title" in any admin
  updater.component('TextField.*.*.Title', MyTextField);
});

To apply context-based transformations, you'll need to know the context of the component you want to customise. To learn this, open your React Developer Tools (see above) window and inspect the component name. The context of the component is displayed between two square brackets, appended to the original name, for example: TextField[TextField.AssetAdmin.FileEditForm.Title]. The context description is hierarchical, starting with the most general category (in this case, "Admin") and working its way down to the most specific category (Name = 'Title'). You can use Injector to hook into the level of specificity that you want.

Customising react components with Injector#

When middleware is used to customise a React component, it is known as a higher order component.

Using the PhotoItem example above, let's create a customised PhotoItem that allows a badge, perhaps indicating that it is new to the gallery.

import React from 'react';
// ...

const enhancedPhoto = (PhotoItem) => (props) => {
  const badge = props.isNew ?
    <div className="badge">New!</div> :
    null;

  return (
    <div>
      {badge}
      <PhotoItem {...props} />
    </div>
  );
};

const EnhancedPhotoItem = enhancedPhoto(PhotoItem);

  <EnhancedPhotoItem isNew size={300} />;

Alternatively, this component could be expressed with an ES6 class, rather than a simple function.

import React from 'react';
// ...

const enhancedPhoto = (PhotoItem) => (
  class EnhancedPhotoItem extends React.Component {
    render() {
      const badge = this.props.isNew ?
        <div className="badge">New!</div> :
        null;

      return (
        <div>
          {badge}
          <PhotoItem {...this.props} />
        </div>
      );
    }
  }
);

When components are stateless, using a simple function in lieu of a class is recommended.

Using dependencies within your react component#

If your component has dependencies, you can add them via the injector using the inject() higher order component. The function accepts the following arguments:

inject([dependencies], mapDependenciesToProps, getContext)(Component);

• [dependencies]: An array of dependencies (or a string, if just one)
• mapDependenciesToProps: (optional) All dependencies are passed into this function as params. The function is expected to return a map of props to dependencies. If this parameter is not specified, the prop names and the service names will mirror each other.
• getContext: A callback function with params (props, currentContext) that will calculate the context to use for determining which transformations apply to the dependencies. This defaults to the current context. This could help when any customisations that may calls for a change (or tweak) to the current context.

The result is a function that is ready to apply to a component.

const MyInjectedComponent = inject(
  ['Dependency1', 'Dependency2']
)(MyComponent);
// MyComponent now has access to props.Dependency1 and props.Dependency2

Here is its usage with a bit more context:

// my-module/js/components/Gallery.js
import React from 'react';
import { inject } from 'lib/Injector';

class Gallery extends React.Component {
  render() {
    const { SearchComponent, ItemComponent } = this.props;
    return (
      <div>
        <SearchComponent />
        {this.props.items.map(item => (
          <ItemComponent title={item.title} image={item.image} />
        ))}
      </div>
    );
  }
}

export default inject(
  ['GalleryItem', 'SearchBar'],
  (GalleryItem, SearchBar) => ({
    ItemComponent: GalleryItem,
    SearchComponent: SearchBar
  }),
  () => 'Gallery.Search'
)(Gallery);

The properties used by inject() are soft-supplied. This means a parent calling a component that uses inject() could choose to overwrite the dependencies which inject() would have otherwise supplied. Here is an example using the above Gallery component with the dependency ItemComponent overwritten by the calling component. We pull in a previously registered PreviewItem to replace the former GalleryItem.

// my-module/js/components/PreviewSection.js
import React from 'react';
import { inject } from 'lib/Injector';

class PreviewSection extends React.Component {
  render() {
    const { Gallery, PreviewItem } = this.props;
    return (
      <div className="preview-section">
        <div className="preview-sidebar">Sidebar here</div>
        <Gallery ItemComponent={PreviewItem}/>
      </div>
    );
  }
}

export default inject(
  ['Gallery', 'PreviewItem']
)(PreviewSection);

Another way to provide context to injector is by using the provideContext HOC, rather than the getContext param in inject().

// my-module/js/components/ContextualSection.js
import React, { Component } from 'react';
import { provideContext, inject } from 'lib/Injector';

class MySection extends Component {
  // ... section code here ...
}

export default compose(
  provideContext('Gallery.Search'),
  inject(['Gallery'])
)(MySection);

Using the injector directly within your component#

On rare occasions, you may just want direct access to the injector in your component. If your dependency requirements are dynamic, for example, you won't be able to explicitly declare them in inject(). In cases like this, use withInjector(). This higher order component puts the Injector instance in context.

import React from 'react';
// ...

class MyGallery extends React.Component {
  render() {
    return <div>
      {this.props.items.map(item => {
        const Component = this.context.injector.get(item.type, 'Reports.context');
        return <Component title={item.title} image={item.image} />;
      })}
    </div>;
  }
}

export default withInjector(MyGallery);

The Reports.context in the second parameter provides a context for the injector to determine which transformations to apply to or remove from the component you're looking to get. More details about transformations below.

Using injector to customise forms#

Forms in the React layer are built declaratively, using the FormSchema API. A component called FormBuilderLoader is given a URL to a form schema definition, and it populates itself with fields (both structural and data-containing) and actions to create the UI for the form. Each form is required to have an identifier property, which is used to create context for Injector when field components are fetched. This affords developers the opportunity provide very surgical customisations.

Updating the form schema#

Most behavioural and aesthetic customisations will happen via a mutation of the form schema. For this, we'll use the updater.form.alterSchema() function.

Injector.transform(
  'my-custom-form',
  (updater) => {
    updater.form.alterSchema(
      'AssetAdmin.*',
      (form) =>
        form.updateField('Title', {
          myCustomProp: true
        })
          .getState()
    );
  }
);

The alterSchema() function takes a callback, with an instance of FormStateManager (form in the above example) as a parameter. FormStateMangaer allows you to declaratively update the form schema API using several helper methods, including:

• updateField(fieldName:string, updates:object)
• updateFields({ myFieldName: updates:object })
• mutateField(fieldName:string, callback:function)
• setFieldComponent(fieldName:string, componentName:string)
• setFieldClass(fieldName:string, cssClassName:string, active:boolean)
• addFieldClass(fieldName:string, cssClassName:string)
• removeFieldClass(fieldName:string, cssClassName:string)

In addition to mutation methods, several readonly methods are available on FormSchemaManager to read the current form state, including:

• getValues(): Returns a map of field names to their current values
• getValue(fieldName:string): Returns the value of the given field
• isDirty(): Returns true if the form has been mutated from its original state
• isPristine(): Returns true if the form is in its original state
• isValid(): Returns true if the form has no validation errors
• isInvalid(): Returns true if the form has validation errors

Adding validation to a form#

Validation for React-rendered forms is handled by the redux-form package. You can inject your own middleware to add custom validation rules using the updater.form.addValidation() function.

Injector.transform(
  'my-validation',
  (updater) => {
    updater.form.addValidation(
      'AssetAdmin.*',
      (values, validator) => {
        if (values.PostalCode.length !== 5) {
          validator.addError('PostalCode', 'Invalid postal code');
        }
      }
    );
  }
);

The addValidation() function takes a callback, with an instance of FormValidationManager (validator in the above example) as a parameter. FormValidationMangaer allows you to manage the validation result using several helper methods, including:

• addError(fieldName:string, message:string)
• addErrors(fieldName:string, messages:Array)
• hasError(fieldName:string)
• clearErrors(fieldName:string)
• getErrors(fieldName:string)
• reset(void)

Using injector to customise redux state data#

Before starting this tutorial, you should become familiar with the concepts of Immutability and Redux.

For example:

newProps = { ...oldProps, name: 'New name' };

is the same as

newProps = Object.assign(
  {},
  oldProps,
  { name: 'New name' }
);

To start customising, you'll need to transform an existing registered reducer, you can find what reducers are registered by importing Injector and running Injector.reducer.getAll()

Injector.transform('customisationName', (updater) => {
  updater.reducer('assetAdmin', MyReducerTransformer);
});

As you can see, we use the reducer() function on the update object to augment Redux state transformations.

Using redux dev tools#

It is important to learn the basics of Redux dev tools, so that you can find out what ACTIONS and payloads to intercept and modify in your Transformer should target.

Most importantly, it helps to understand the "Action" sub-tab on the right panel (bottom if your dev tools is small), as this will be the data your Transformer will most likely receive, pending other transformers that may run before/after your one.

Structuring a transformer#

We use currying to supply utilities which your transformer may require to handle the transformation.

• originalReducer - reducer callback which the transformer is customising, this will need to be called in most cases. This will also callback other transformations down the chain of execution. Not calling this will break the chain.
• getGlobalState - A function that gets the state of the global Redux store. There may be data outside the current scope in the reducer which you may need to help determine the transformation.
• state - current state of the current scope. This is what should be used to form the new state.
• type - the action to fire, like in any reducer in Redux. This helps determine if your transformer should do anything.
• payload - the new data sent with the action to mutate the Redux store.

const MyReducerTransformer = (originalReducer) => (globalState) => (state, { type, payload }) => {
  switch (type) {
    case 'EXISTING_ACTION': {
      // recommended to call and return the originalReducer with the payload changed by the transformer
      return originalReducer(/* ... */);
    }

    case 'OVERRIDE_EXISTING_ACTION': {
      // could omit the originalReducer to enforce your change or cancel the originalREducer's change
      return originalReducer(/* ... */);
    }

    default: {
      // it is important to return the originalReducer with original redux parameters.
      return originalReducer(state, { type, payload });
    }
  }
};

A basic transformation#

This example we will illustrate modifying the payload to get different data saved into the original reducer.

We will rename anything in the breadcrumbs that is displaying "Files" to display "Custom Files" instead.

const MyReducerTransformer = (originalReducer) => (getGlobalState) => (state, { type, payload }) => {
  switch (type) {
    case 'SET_BREADCRUMBS': {
      return originalReducer(state, {
        type,
        payload: {
          breadcrumbs: payload.breadcrumbs.map((crumb) => (
            (crumb.text === 'Files')
              ? { ...crumb, text: 'Custom Files' }
              : crumb
          )),
        },
      });
    }
    default: {
      return state;
    }
  }
};

Using the globalState#

Accessing the globalState is easy, as it is passed in as part of the curried functions definition.

export default (originalReducer) => (getGlobalState) => (state, { type, payload }) => {
  const baseUrl = globalState.config.baseUrl;

  switch (type) {
    /* ... cases here ... */
    default: {
      // ...
    }
  }
};

Setting a different initial state#

We can easily define a new initial state by providing the state param with a default value. It is recommended to keep the call for the original initialState for your initialState then override values, so that you do not lose any potentially critical data that would have originally been set.

const MyReducerTransformer = (originalReducer) => () => (state, { type, payload }) => {
  if (typeof state === 'undefined') {
    return {
      ...originalReducer(state, { type, payload }),
      myCustom: 'initial state here',
    };
  }
  return state;
};

Cancelling an action#

There are valid reasons to break the chain of reducer transformations, such as cancelling the Redux store update. However, like an original reducer in redux, you will still need to return the original state.

export default (originalReducer) => (getGlobalState) => (state, { type, payload }) => {
  switch (type) {
    case 'CANCEL_THIS_ACTION': {
      return state;
    }
    default: {
      return state;
    }
  }
};

Calling a different action#

You could manipulate the action called by the originalReducer, there isn't an example available but this block of code will present the theory of how it can be achieved.

export default (originalReducer) => (getGlobalState) => (state, { type, payload }) => {
  switch (type) {
    case 'REMOVE_ERROR': {
      // we'd like to archive errors instead of removing them
      return originalReducer(state, {
        type: 'ARCHIVE_ERROR',
        payload,
      });
    }
    default: {
      return state;
    }
  }
};

Extensions are only as good as the code they're extending#

An important point to remember about these types of deep customisations is that they all depend heavily on the core code they're modifying to follow specific patterns. The more the core code makes use of Injector the easier it will be for third party developers to extend. Conversely, if the core is full of hard-coded component definitions and statically written queries, customisation will be at best less surgical and at worst, not possible.